Referring first to FIG. 27, there is illustrated a commonly-used hot water supply apparatus of the heat pump type. This conventional hot water supply apparatus is made up of a tank unit 71 having a hot water storage tank 70, and a heat source unit 73 having a refrigerant circuit 72. The refrigerant circuit 72 is provided with a compressor 74, a condenser 75 which is a water heat exchanger, a receiver 76, an expansion valve 77, and an evaporator 78. On the other hand, the tank unit 71 is provided with the hot water storage tank 70 and a circulation path 79. A pump 80 and a heat exchange path 81 are inserted in the circulation path 79. In this case, the heat exchange path 81 is formed by the water heat exchanger 75.
Accordingly, when the compressor 74 is activated while the pump 80 is activated (operated), stored water (warm water) starts flowing into the circulation path 79 from a water intake port provided at the bottom of the hot water storage tank 70 and circulates through the heat exchanging path 81. At this time, the warm water is heated (boiled) by the condenser (water heat exchanger) 75 and is directed back to the upper part of the hot water storage tank 70 from a hot water supply port. Hereby, high temperature warm water is stored in the hot water storage tank 70.
Hitherto, as the refrigerant of a refrigerant circuit of the type describe above, various refrigerants, such as dichloro difluoro methane (R-12), chloro difluoro methane (R22) et cetera, have been used. However, alternative refrigerants, such as 1,1,1,2-tetra fluoro ethane (R-134a) et cetera, are now being used to cope with problems, e.g., ozone layer destruction and environmental pollution. However, the refrigerant R-134a is still problematic because it exhibits a high global warming potential. Accordingly, use of natural refrigerants free from these environmental problems has been recommended. The fact that super-critical refrigerants such as carbon dioxide gas are useful as natural refrigerant has been known in the art. By the term “super-critical refrigerant” used here is meant a refrigerant which performs a refrigerating cycle by compression to above a critical pressure in the compressor.